This application pertains to the art of manufacturing plastic conduits and more particularly to the art of manufacturing corrugated, foldable plastic tubing. The invention is particularly applicable to corrugated, plastic drainage tubing and will be described with reference thereto. It is to be appreciated, however, that the invention has broader applications such as manufacturing septic or leach bed tubings, subterranean conduits, other fluid conveying channels, pipes for shielding electrical cables, and the like.
In the past, most corrugated drainage tubing has been circular in cross section. The circular tubing is generally manufactured in a linear production line assembly which produces a single length of tubing. Commonly, a plastic extruding machine fitted with a circular extrusion die receives pellets of a thermoplastic material and extrudes a plastic sleeve of softened plastic through the die. A continuous blow molding assembly is positioned closely adjacent the extruder die to shape the extruded sleeve into circular corrugated tubing. Commonly, the blow molding assembly includes two runs of semicircular mold blocks which travel linearly along parallel paths adjacent the extruder die as pneumatic pressure forces the sleeve into conformity the interior surface of the blocks. The mold blocks draw heat out of the tubing as they travel away from the extrusion die. When the plastic tubing cools sufficiently to retain its molded shape, the mold blocks separate and return to the initial position.
After the tubing is molded, it is generally cooled with a water bath. If the tubing is to be used for drainage tubing, the continuous length of tubing is commonly conveyed adjacent a slotting saw which cuts a series of thin slots into the side walls of the tubing. If the tubing is to be used for septic tubing, the tubing is commonly conveyed through a drilling machine which drills round holes about three-quarters of an inch in diameter into the side walls of the tubing. If the tubing is to be used for conveying fluid, generally, it is neither slotted nor drilled. On some occasions, the assembly line may include a device for wrapping the drilled or slotted tubing with a filter material which inhibits small particles from entering the tubing. The round corrugated tubing is commonly cut in ten foot length or coiled in 250 foot rolls.
The present assignee has previously developed an arched corrugated conduit which has improved strength characteristics. This, arched conduit is described in U.S. Pat. No. 3,861,925, issued Aug. 8, 1972. When manufacturing this arched conduit, only a single length of conduit is produced on each assembly line. The present assignee subsequently developed the foldable, arched conduit which is described in the U.S. Pat. No. 4,245,924, issued Jan. 20, 1981. The foldable conduit is an improvement over the prior arched and circular conduits in that two or more lengths of conduit are manufactured concurrently on a single production line. Previously, the present assignee has filed U.S. patent application Ser. No. 162,006, filed June 23, 1980, now U.S. Pat. No. 4,357,190 and patent application Ser. No. 240,602, filed Mar. 4, 1981, now U.S. Pat. No. 4,374,079 both directed to methods and apparatus for manufacturing foldable tubing. Although the method and apparatus for manufacturing plastic tubing disclosed in these earlier applications has been reasonably successful, the present invention increases the speed at which the conduit is manufactured and improves the quality and appearance of the conduit.